The present invention pertains to a device for the separation by flotation of nonmiscible liquids. The invention focuses more specifically on the treatment of water polluted by hydrocarbons.
It is known to treat such mixtures of water and hydrocarbons by flotation either with dissolved air (D.A.F. system, "dissolved air flotation") or with induced air or gas (I.A.F. systems, "induced air flotation"). In I.A.F. devices, the induction of air or gas is generated either by means of injectors or by means of rotor-stator type turbines. The device in accordance with the present invention pertains to this latter category. Thus, flotation devices of the I.A.F. type are known which operate by means of turbine-induced injection of air or gas, which devices are constituted of a cylindrical tank with one or more passive zones in which the tranquilization or decantation of the oils takes place, and with one or more active zones in which a rotor/stator unit operates as a flotation-air injection device, and which discharge into one or more collector chambers receiving the froth formed in the active compartments. Information on these devices can be found in the manufacturers' bulletins and in various patents, e.g., French Patent No. 2,605,898.
The role of the turbines is to induce, via rotation of the rotor, a depression in the stack above them, thereby drawing in the covering gas and directing it against the stator, thereby creating around the stator the cloud of gas which will generate the flotation. These turbines are simply constituted, in accordance with the prior art, of a rotor with mobile blades inside the cage of a stator with fingers of rectangular section. These units have a certain number of drawbacks:
a) instability of the flow rate of the air or gas due to the rupture of the diphasic flow profile in the rotor's stack; the rotor rotates in a diphasic water/air medium, which results in a considerable fluctuation in the electrical power absorbed;
b) periodic ascension of the vortex up to the air-induction point, leading to water being directed onto the froths, thereby inducing their partial redissolution;
c) the flotation effect is not assured with turbines handling flow rates greater than 50 m.sup.3 /hour since the turbines only act as agitators because the depression created by the rotor is not sufficient to carry the covering gas to the stator; and
d) impossibility of varying the ratio between the water flow rate and the air or gas flow rate.
For these reasons, even the operation of these units at the pilot scale is acceptable, when the equipment described in the prior art is produced at industrial scale the flotation effect is not satisfactory.
The skimming chutes in accordance with the prior art are self-stabilizing. Nevertheless, they are not equipped with any possibilities for regulating their immersion, their skimming perimeter is very small, their horizontality is very difficult to maintain and their flotation is uncertain, i.e., the chute can partially emerge from the water or leak; the result is that the skimming of the froths is very imperfect and the device becomes totally unstable after several weeks of operation.